I received a final notice letter through door from an agent this was the first I heard of marstons having taken on my csa debt.
I did not receive any prior notice from marston that they wanted payment.
I have payed the debt and £75 admin fee but they say the additional £333 is payable because it moved to enforcement stage they say they posted compliance letter though I did not receive it. I have written to them at stage 1 of the complaints procedure and they say basically tough its not there fault that I did not get the letter and it is still owed, what can I do to avoid having to pay this exorbitant additional fee?

I'll need to look at the legislation to see if the statutory fee schedule has been applied to the recovery of child maintenance. Ive been told it has but to be honest, I like to see the statute book before I comment further, but for the moment, I will assume it has been implemented.

You dot need to prove the agent has not attended your address, which the law introduces the £235 fee (Not sure where you got the £333 from)] is regulation 5(1)(b) of the Taking Control of Goods (Fees) Regulations 2014. If the bailiff asserts he attended your address then he must make a sworn statement as the test of proof the attendance took place and when. If the bailiff refuses to take the test then you do not have to pay the attendance fee.

If you are absolutely certain they bailiff has not attended, then you make a sworn affidavit proving this, and the CSA should withdraw enforcement action, and if they fail to follow regulations, you can have the Parliamentary Ombudsman intervene.
Here is more about bailiffs and the CSA

Thank you for response.
I should clarify a few points.
The agent called at my property while I was out and posted a final notice letter with the total debt plus £75 admin fee plus £333 (235+7.5%) enforcement fee.
I was aware of the debt but not Marston involvement, I received no notice of there intent (compliance stage?) .
I had no quibble about paying the debt and have done so.
I have challenged the fact I received no seven day notice letter straight to enforcement stage. They say they sent the letter standard mail.
Is it right that if the letter did not get to me thus giving me time to agree a payment plan, that they can still enforce payment of fee, can they do this as part of original liability order or would they need a new order?

The % only applies to debts over £1500 being recovered.
The law says you must be 'GIVEN' a statutory notice of enforcement at least seven clear days excluding public holidays and Sundays before a bailiff starts enforcement.

The law is Regulation 6 of the Taking Control of Goods Regulations 2013 and Paragraph 7(1) of Schedule 12 of the Tribunals, Courts and Enforcement Act 2007. The allowed methods giving the notice are prescribed in Regulation 8 (1)(e).

If the notice was not given to you at least seven clear days excluding Sundays and public holidays then all enforcement action that follows is not compliant with enforcement regulations. You can start the formal complaints procedures as shown here

Ask the bailiff company for a tracking number for the Notice of enforcement. If they cannot produce one then use Section 7 of the Interpretation Act 1978 by making a sworn declaration you have not been given the notice. Here is a template Statutory Declaration. It also revokes all the statutory fees the bailiff has charged you. Once you have sworn it in before a solicitor, give copies by email to the council, the bailiff company and by text message to the bailiff's mobile. You can get the council CEO's email address here. Ask the council to cease and disist from continiuing non-compliant enforcement action. If you get fobbed off, speak to the Ombudsman.

The 'GIVEN' is the main point for me, they say they sent it I say that I didn't receive it. Wouldn't they have to prove I received the letter.
I shall write to them quoting the legislation.
Thank you for advice.